fix: use `--upgrade` rather than `--ignore-installed` to upgrade pip

[mayeut]

This removes left over files when upgrading pip.

See #254 for more information on the issues fixed by this PR:

• https://github.com/actions/setup-python/issues/785
• https://github.com/actions/setup-python/issues/713

Extends #254 by adding a test & fixing installers/macos-pkg-setup-template.sh

Closes #254